In recent years, in the field of radio communication, frequency resources have become increasingly tight. Thus, high-efficient transmission using a digital modulation scheme is widely used. A radio communication apparatus using a digital modulation scheme typically includes a power amplifier used to amplify and output a modulated signal.
When the power amplifier is operated substantially at the saturation region, the usage efficiency of power is higher. However, when the power of an input signal is large, nonlinear distortion of an output signal occurs and thus the frequency spectrum becomes wider, causing leakage of the output signal power to adjacent channels. Accordingly, the radio communication apparatus includes a distortion correcting unit which corrects a distortion of the signal outputted from the power amplifier and thereby reduces a leakage of power to adjacent channels.
FIG. 1 is a schematic functional block diagram of a power amplifier and a distortion correcting unit.
FIG. 1 illustrates a power amplifier 10 used to amplify an input signal power and a distortion correcting unit used to correct a distortion of a signal outputted from the power amplifier 10. The distortion correcting unit includes a pre-distortion section 20, a delay adjusting section 30, and a delay amount estimating section 40.
The delay adjusting section 30 and the delay amount estimating section 40 receive an input signal S(t) and a feedback signal S(t−τ) outputted from the power amplifier 10. The delay amount estimating section 40 estimates a delay amount τ of the feedback signal relative to the input signal, and the delay adjusting section 30 delays an input signal by the delay amount τ and sends the signal to the pre-distortion section 20. As a result, the input signal S(t) and the feedback signal S(t−τ) are inputted to the pre-distortion section 20 at the same time. Further, the pre-distortion section 20 calculates a distortion correcting characteristic based on the input signal and feedback signal, and the input signal is multiplied by the calculated correcting characteristic, whereby the distortion of the signal outputted from the power amplifier 10 is reduced.
In radio communication systems, the signal power and bandwidth are regulated according to the user's system usage and the like, so the delay amount of the feedback signal outputted from the power amplifier 10 also fluctuates. Accordingly, in order to improve the distortion correcting performance by the pre-distortion section 20, the delay amount τ of the feedback signal relative to the input signal is preferably estimated with high accuracy.
Delay amount estimating methods hitherto proposed include: one for calculating a correlation value between the input signal and feedback signal while varying the delay amount, and determining as an optimal delay amount, a delay amount τ at which a maximum correlation value is provided (for example, refer to Japanese Patent Laid-Open No. 2001-189685); and one for using a delay amount obtained by varying, by a specific amount, a delay amount τ at which a maximum correlation value is provided (for example, refer to Japanese Patent Laid-Open No. 9-64857).
FIG. 2 is a view illustrating a relationship between the delay amount of the feedback signal relative to the input signal and the correlation value between the input signal and feedback signal.
Referring to FIG. 2, the abscissa represents the delay amount of the feedback signal relative to the input signal, and the ordinate represents the correlation value between the input signal and feedback signal.
In the delay amount estimating section 40 illustrated in FIG. 1, a correlation calculating section 42 calculates a correlation value between the input signal and feedback signal for each delay amount τ while varying, by a specific amount, the delay amount τ of the feedback signal relative to the input signal; and a maximum correlation value determining section 41 determines, as an optimal delay amount, a delay amount τm at which a maximum correlation value is provided.
However, in a region where the maximum correlation value is provided, the variance of a correlation value relative to the variance of the delay amount is smaller and thus calculation errors have much effect, causing a variation in correlation value. Consequently, a maximum correlation value cannot be determined with high accuracy, so it is difficult to determine an optimal delay amount when only a maximum correlation value is used.
The present invention addresses the above problem by providing a delay amount estimating apparatus and a signal transmitting apparatus in which a delay amount of a feedback signal relative to an input signal is estimated with high accuracy.